1. Field of the Invention
One aspect of the present invention relates to a casing of an electronic apparatus with improved reliability of joints, and a supporting structure of a circuit board on which electronic components are mounted.
2. Description of the Related Art
With sophistication, multi-functionalization, etc., of instruments mounted with electronic apparatuses, the number of electrical joints which joins a circuit board and an electronic apparatus becomes an extremely large number. As a result, ensuring of joint reliability is not easy.
In electronic apparatuses, there is a tendency that thermal stress resulting from a difference between coefficients of linear expansion of a casing and a circuit board, and stress resulting from external load become large especially around a boss portion. Therefore, when the circuit board is fastened and fixed to the casing via the boss portion, the load of electronic components on the circuit board in a vicinity of the boss portion increases. Especially, more large-sized electronic components are there in the vicinity of a middle of the circuit board. This problem is actualized. Hereinafter, a cause of deterioration of reliability of a joint will be specifically described with reference to FIG. 13. A circuit board 13 is fixed to casing boss portions 27, 28, and 29. When the circuit board 13, the upper casing 11, and the lower casing 12 are fixed at the boss portions 27, 28, and 29, and the stress of the circuit board 13 around the boss portions 27 and 28 by temperature fluctuation and the 29 becomes large due to the difference between the coefficients of linear expansion of the circuit board 13, and the upper casing 11 or the lower casing 12.
Also, in a case where external load is generated, since deformation of the upper casing 11 and the lower casing 12 is transmitted to the circuit board via the boss portions 27, 28, and 29, the stress of the circuit board 13 around the boss portions 27 and 28 and the 29 becomes large. In particular, the tendency that the load of electronic components 19 in the vicinity of the boss portion 27 increases is observed in the middle of the circuit board 13.
A method of fastening a shielding plate and a boss portion of a casing is suggested in connection with a circuit board (see JP-A-9-8488 (FIG. 2), for instance).
This method is a method of arranging a solder pattern, and jointly fastening the circuit board having a solder portion formed thereon and the shielding plate to the boss portion of the casing while the solder portion is interposed therebetween.
In the invention disclosed in JP-A-9-8488, the circuit board is soldered to the casing, and electrical connection is secured. Thus, thermal stress can be reduced. However, the concentration of stress caused around an attaching portion by external load onto an electronic component mounted on the circuit board cannot be reduced.